linux-kernel - Re: [PATCH v10 08/50] x86/fault: Add helper for dumping RMP entries

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Message-ID: <20231115160852.GDZVTtFHB0+HpVZnpG@fat_crate.local>
Date:   Wed, 15 Nov 2023 17:08:52 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org,
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        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
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        pankaj.gupta@....com, liam.merwick@...cle.com,
        zhi.a.wang@...el.com, Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v10 08/50] x86/fault: Add helper for dumping RMP entries

On Mon, Oct 16, 2023 at 08:27:37AM -0500, Michael Roth wrote:
> +/*
> + * Dump the raw RMP entry for a particular PFN. These bits are documented in the
> + * PPR for a particular CPU model and provide useful information about how a
> + * particular PFN is being utilized by the kernel/firmware at the time certain
> + * unexpected events occur, such as RMP faults.
> + */
> +static void sev_dump_rmpentry(u64 dumped_pfn)

Just "dump_rmentry"

s/dumped_pfn/pfn/g

> +	struct rmpentry e;
> +	u64 pfn, pfn_end;
> +	int level, ret;
> +	u64 *e_data;
> +
> +	ret = __snp_lookup_rmpentry(dumped_pfn, &e, &level);
> +	if (ret) {
> +		pr_info("Failed to read RMP entry for PFN 0x%llx, error %d\n",
> +			dumped_pfn, ret);
> +		return;
> +	}
> +
> +	e_data = (u64 *)&e;
> +	if (e.assigned) {
> +		pr_info("RMP entry for PFN 0x%llx: [high=0x%016llx low=0x%016llx]\n",
> +			dumped_pfn, e_data[1], e_data[0]);
> +		return;
> +	}
> +
> +	/*
> +	 * If the RMP entry for a particular PFN is not in an assigned state,
> +	 * then it is sometimes useful to get an idea of whether or not any RMP
> +	 * entries for other PFNs within the same 2MB region are assigned, since
> +	 * those too can affect the ability to access a particular PFN in
> +	 * certain situations, such as when the PFN is being accessed via a 2MB
> +	 * mapping in the host page table.
> +	 */
> +	pfn = ALIGN(dumped_pfn, PTRS_PER_PMD);
> +	pfn_end = pfn + PTRS_PER_PMD;
> +
> +	while (pfn < pfn_end) {
> +		ret = __snp_lookup_rmpentry(pfn, &e, &level);
> +		if (ret) {
> +			pr_info_ratelimited("Failed to read RMP entry for PFN 0x%llx\n", pfn);

Why ratelmited?

No need to print anything if you fail to read it - simply dump the range
[pfn, pfn_end], _data[0], e_data[1] exactly *once* before the loop and
inside the loop dump only the ones you can lookup...

> +			pfn++;
> +			continue;
> +		}
> +
> +		if (e_data[0] || e_data[1]) {
> +			pr_info("No assigned RMP entry for PFN 0x%llx, but the 2MB region contains populated RMP entries, e.g.: PFN 0x%llx: [high=0x%016llx low=0x%016llx]\n",
> +				dumped_pfn, pfn, e_data[1], e_data[0]);
> +			return;
> +		}
> +		pfn++;
> +	}
> +
> +	pr_info("No populated RMP entries in the 2MB region containing PFN 0x%llx\n",
> +		dumped_pfn);

... and then you don't need this one either.

> +}
> +
> +void sev_dump_hva_rmpentry(unsigned long hva)
> +{
> +	unsigned int level;
> +	pgd_t *pgd;
> +	pte_t *pte;
> +
> +	pgd = __va(read_cr3_pa());
> +	pgd += pgd_index(hva);
> +	pte = lookup_address_in_pgd(pgd, hva, &level);

If this is using the current CR3, why aren't you simply using
lookup_address() here without the need to read pgd?

> +
> +	if (pte) {

	if (!pte)

Doh.

> +		pr_info("Can't dump RMP entry for HVA %lx: no PTE/PFN found\n", hva);
> +		return;
> +	}
> +
> +	sev_dump_rmpentry(pte_pfn(*pte));
> +}
> +EXPORT_SYMBOL_GPL(sev_dump_hva_rmpentry);

Who's going to use this, kvm?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette